Femoral prosthesis with wedge having opposed tapers

ABSTRACT

A prosthesis having a proximal portion of a stem formed as a wedge having opposed tapers such that prostheses can be thrust into a medullary canal and achieve fixation without unduly stressing the cortical bone remaining in the femur. Ribs secure the prosthesis against medial-lateral motion. Simultaneously, a degree of flexibility is provided in an anterior-posterior direction. A slotted distal portion of the stem is flared to enhance fixation distally.

BACKGROUND OF THE INVENTION

Our invention relates to femoral prostheses and in particular toprostheses adapted for non-cemented fixation in the medullary canal of afemur.

Various types of femoral prostheses are known and are used for surgicalreconstruction of a femur. In general, these prostheses comprise aball-shaped head mounted at an anatomical angle on a shank. The shankcan be thrust into a medullary canal of a femur to mount the prosthesison a resected surface of the femur. Various means of fixation are known,including bone cement, or porous areas on the prosthesis which promotebony ingrowth, or shoulders proximal to the head for preventing theprosthesis from wedging into the medullary canal.

In femoral prostheses which are particularly adapted for non-cementedapplications, initial fixation and subsequent bony ingrowth areimportant features. Micromotion of the prosthesis may result in reducedfixation, compromising the long-term stability of the prosthesis. At thesame time, variation in human anatomy makes it difficult to provide awide range of tightly fitting sizes for femoral prostheses. Thus, thereremains a continuing need for improvement in the design of non-cementedprostheses which provide for firm initial fixation while simultaneouslyavoiding the possibility of overloading the surrounding bone.

SUMMARY OF OUR INVENTION

Our invention relates to prostheses which are particularly adapted fornon-cemented fixation. We have designed a prosthesis having a proximalportion of a stem formed as a wedge having opposed tapers. Thisstructure will be more fully explained below. The result of the opposedtapers is that prostheses can be thrust into a medullary canal andachieve fixation without unduly stressing the cortical bone remaining inthe femur. In addition, we have designed a prosthesis having a distalportion of the stem adapted for initial fixation. We have provided ribswhich secure the prosthesis against medial-lateral motion.Simultaneously, a degree of flexibility is provided in ananterior-posterior direction to remove or to diminish stress pain. Wepropose a slight but significant flair in a slotted distal portion ofthe stem to enhance fixation distally. To resist motion in theanterior-posterior direction, we have provided four areas on the lateralside with different fixation characteristics: A porous area proximallyfor long-term fixation; a ribbed bone for immediate rigid fixation, azone with a chamfered rib for intermediate fixation; and a distal zonewith a medial-lateral slot which is relatively flexible. To resistmotion in the medial-lateral direction, we have provided three areas onthe anterior and posterior sides with different fixationcharacteristics: A porous area proximally which includes the wedgestructure for immedate fixation; an intermedaiate portion; and a distalportion with ribs which are increasingly prominent distally.

It is an object of our invention, therefore, to provide a femoralprosthesis with structure which enhances non-cemented fixation.

It is another important object of our invention to provided such aprosthesis with features which reduce stresses in proximal corticalbone.

Another important object of our invention is to provide a femoralprosthesis having distal fixation.

It is yet a further object of our invention to provide a prosthesis withdistal fixation which is enhanced in the medial-lateral plane.

A further object of our invention is to provided a prosthesis withflexibility in the anterior-posterior plane combined with distalfixation.

These and other objects and advantages of our invention would beapparent from the following detailed description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a femoral prosthesis according to our presentinvention, with attached femoral head.

FIG. 2 is lateral plan view of the prosthesis stem of FIG. 1.

FIG. 3 is an posterior plan view of the stem of FIG. 1.

FIG. 4 is a prospective view of a double wedge of the proximal portionof the stem of FIG. 1.

FIG. 5 is a top plan view of the wedge of FIG. 4.

FIG. 6 is a left side plan view of the wedge of FIG. 4.

FIG. 7 is a front plan view of the wedge of FIG. 4.

FIG. 8 is a right plan view of the wedge of FIG. 4.

FIG. 9 is a cross-section of a distal portion of the stem of FIG. 1taken along line 9--9.

FIG. 10 is anterior plan view of the stem of FIG. 1 showing ourpreferred location for porous coating.

FIG. 11 is a lateral plan view of the stem of FIG. 10.

FIG. 12 is an posterior plan view of the stem of FIG. 10.

FIG. 13 is an enlarged partial plan view of the stem of FIG. 2, showinga flared distal slot.

DETAILED DESCRIPTION OF OUR PREFERRED EMBODIMENT

In referring to the accompanying drawings, like numerals will refer tolike parts throughout the description. In FIG. 1, a femoral prosthesis10 is illustrated. The femoral prosthesis 10 comprises a stem 12 adaptedto be inserted into the medullary canal of a resected femur and aspherical articulating head 14 mounted detachably on a neck 16. The stem12 has a proximal body portion 18 and a distal portion 20. The distalportion is relatively straight on a lateral side 36 with a slightchamfer 22 proximally. Medially, the proximal portion is concave curved24. Viewed laterally, as in FIG. 2, an anterior side 28 and posteriorside 26 are slightly flared proximally. In addition, on the anteriorproximal side, there is a double wedge structure 30 which will be morefully described below. Between the proximal portion and the neck 16,there is a collar 32 which extends anteriorly, medially and posteriorlyfrom the proximal portion.

The distal portion 20 of the stem is relatively straight, but isslightly tapered inwardly from the proximal portion to a distal end 34.On the lateral side 36, we have provided a rib 38 which extends near thedouble wedge 30 to the distal tip 34. In a proximal half 40 of thisridge 38, which comprises a first zone, the ridge is of relativelyrectangular cross-section, providing sharp edges which could gripsurrounding cancellous bone. In a distal part 42 of the ridge, we haveprovided anterior and posterior chamfers, 46, 44 respectively. Thesechamfers flare out from the ridge 40, widening the ridge and softeningits outline. This tends to fill more of the distal meduallary canal butreduces resistance to motion in the anterior-posterior plane in a secondzone. In addition, we have provided a slot 47 which extends in themedial-lateral plane from the distal end 34 about half of the distancefrom the distal end 34 to the beginning of the chambers 44, 46, whichcomprises a distal zone. The slot 47 terminates at a stress relief bore48. The slot 47 permits increased flexing of the prosthesis in anteriorand posterior directions, reducing point pains which are frequentlyexperienced by patients with femoral prostheses.

On both the anterior and posterior sides 28, 26, in the distal portionof the stem, we have provided ribs 50, 52 with substantially rectangularcross-sections extending from distal tip 34 approximately half of theextent of the stem from the distal tip to the collar 32. The ribs 50, 52blend into the stem 12 at their proximal ends. They are, therefore, mostprominent distally and become flatter proximally.

As can be seen in FIG. 9, corners of the distal portion of the stem havechamfers 54, 56, 58 and 60 which generally round the outline of thestem, but which still provide relatively sharp corners which will cutinto surrounding cancellous bone as the stem is inserted into themedullary canal. The chamfers 44, 46 on the lateral rib 38 can also beseen in the cross-section of FIG. 9.

We will now describe in detail the double wedge feature of the proximalportion of our stem. An abstract figure of the double wedge 30 can beseen in FIGS. 4 through 8. The double wedge, as seen in FIG. 4, presentsa wide distal blade 62 which is the distal extent of the double wedgefeature. The distal blade 62 begins a plane 64 of a first wedge or taperwhich extends proximally and inclines away from the body of the proximalportion of the stem. The plane 64 terminates at a vertex 66. The plane64 also defines a second wedge or taper which is broad distally at theblade 62 and narrow proximally at the vertex 66. The edge 62 terminatesat a medial vertex 68 and a lateral vertex 70. These vertices 68, 70 arealso vertices of similar inclined wedge planes, a medial wedge plane 72and a lateral wedge plane 74. Each of these planes 72, 74 intersect atthe proximal vertex 66. A medial edge 76 of the medial wedge plane 72and a lateral edge 78 of the lateral wedge plane 74 flare away from eachother from their respective vertices 68, 70 from distal to proximallocation. The medial edge 76 and lateral edge 78 define a third taperwhich is broader proximally than it is distally. This structure, whichcan also be seen in plan view in FIGS. 5, 6, 7, and 8, creates opposedwedges.

As the stem of the prosthesis is thrust into the medullary canal, thedistal edge 62 will first encounter bone. As the beginning of a wedge,this edge is wide and does not rapidly increase the stress insurrounding cortical bone. As the stem is thrust further into themedullary canal and the bone progresses from the edge 62 toward thevertex 66, the wedge plane 64 will press further and further into thesurrounding cancellous and cortical bone, increasing fixation. At thesame time, the width of this wedge plane 64 will decrease, allowing thestresses to diminish. Simultaneously, the flared features of the medialand lateral wedge planes 72, 74 will tend to fill more of the medullarycanal.

Before applying porous coating to the proximal portion of the stem, weforce open the slot 46 as seen in FIG. 13. The slot itself is on theorder of 3 plus or minus 1 millimeters wide. with the additional flair,the ends of the prosthesis are displaced away from each other anadditional 3 plus or minus 1 millimeters. This is in effect a spring atthe distal end of the prosthesis, which more completely fills themedullary canal. When the prosthesis is heated in connection with theprocess of applying a porous coating, stresses from flaring the slotwill be relieved and the flair set. We anticipate that the slot will notbe compressed beyond the original dimensions of the slot when inserted.There will be variability because of the flair, but there should alwaysbe a gap at the distal end of the stem when the prosthesis is correctlyimplanted.

FIGS. 10, 11 and 12 illustrate the location of porous areas provided inour prosthesis. Although preferably comprised of porous titanium, theseareas define a geometry more clearly seen in FIGS. 1, 2 and 3 anddescribed above. A proximal porous portion 80 for the anterior side,seen in FIG. 10, would therefore, have the opposed wedge geometrydescribed above. The proximal posterior porous area 82, seen in FIG. 12,would be of approximately equal area to the portion 80 but, in ourpreferred embodiment, would not have the opposed wedge configuration,although such a structure could be provided without departing from theteachings of our invention. A porous zone 84 is also provided medially,as is a porous zone 86 on the lateral side of our prosthesis. The porousportions 80, 82 and zones 84, 86 provide long-term proximal fixation assurrounding bone heals into the porous material.

It will be apparent from the foregoing description that our prosthesisprovides different fixation characteristics in both theanterior-posterior and the medial-lateral direction, using variousportions and zones along the length of the prosthesis. To resist motionin the anterior-posterior direction, we have described four areas orzones, principally on the lateral side. Proximally, there is a porouszone which provides long-term fixation. In a first intermediate zone, aprominent ridge provides immediate rigid fixation. In the second, moredistal, intermediate zone, the ridge is chamfered and more motion ispossible, although there is substantial resistance to motion. In adistal zone, the chamfered ridge coupled with the slot allows moremotion and decreases point stress. To resist motion in themedial-lateral direction, we have described a proximal portion which hasthe characteristics of both a porous area for long-term fixation and anopposed wedge for immediate fixation without unduly high stress. Anintermediate portion separates this proximal porous portion from adistal portion. In the distal portion, the ribs taper, becomingincreasingly prominent distally, thereby providing an increasedresistance to medial-lateral motion towards the distal end of theprosthesis.

Our invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore considered in all aspects as illustrative andnot restrictive, the scope of our invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

We claim as our invention:
 1. An implantable prosthesis comprisingarticulating head; anda stem connected to said head for mounting saidprosthesis on a bone of a patient, said stem having integrally fixed onat least one side thereof a bone engaging wedge structure having opposedtapers, said wedge structure having a first tapered surface commencingadjacent said side distally and rising away from said side proximallyand which is broad distally and narrow proximally, said surface lyinggenerally adjacent bone when said prosthesis is implanted.
 2. Theimplantable prosthesis according to claim 1 wherein the wedge structurefurther comprisesa second tapered surface between said first taperedsurface and said side of said stem, said second tapered surface forminga first junction between said second surface and said side and a thirdtapered surface between said first tapered surface and said side of saidstem, said third tapered surface forming a second junction between saidthird tapered surface and said side, said first and second junctionsbeing spaced apart from each other a distal distance at least the distalbreadth of said first surface and a proximal distance greater than saiddistal distance.
 3. The implantable prosthesis according to claim 2wherein said first tapered surface is a plane.
 4. The implantableprosthesis according to claim 3 wherein said second and third taperedsurfaces are planes.
 5. The implantable prosthesis according to claim 4,said stem further comprisingmeans for resisting motion inanterior-posterior directions, said means comprisingporous fixationmeans in a first proximal zone on the stem; means for providingimmediate, substantially rigid fixation in a first intermediate zone onthe stem, substantially adjacent said proximal zone; means for providingimmediate, moderate fixation in a second intermediate zone on the stem,substantially adjacent distally from said first intermediate zone; andmeans for providing immediate, anteriorly-posteriorly flexible fixationin a distal zone on the stem.
 6. The implantable prosthesis according toclaim 5 wherein the rigid fixation means comprise a ridge extending froma proximal end to a distal end of said first intermediate zone.
 7. Theimplantable prosthesis according to claim 6 wherein the moderatefixation means comprise chamfered anterior and posterior edges on saidridge, said ridge and said edges extending from a proximal end to adistal end of said second intermediate zone.
 8. The implantableprosthesis according to claim 7 wherein the flexible fixation meanscomprise a slot extending in a medial-lateral plane in said distal zone.9. The implantable prosthesis according to claim 8 wherein the slot isflared.
 10. The implantable prosthesis according to claim 9 wherein themeans for resisting motion in anterior-posterior directions is on alateral side of said stem.
 11. The implantable prosthesis according toclaim 10 said stem further comprisingmeans for resisting motion inmedial-lateral directions, said means comprising porous fixation meansin a second proximal zone on the stem; immediate fixation means in azone on the stem distal from said second proximal zone, said immediatefixation means providing increasing resistance to medial-lateral motionfrom a proximal end of said distal zone to a distal end of said distalzone.
 12. The implantable prosthesis according to claim 11 wherein theimmediate fixation means comprise a ridge tapering from said proximalend of said distal zone to said distal end of said distal zone.